60 INVOLUTION OF MAMMALIAN MOLAR TKKTH 



Mechanical Relations of tlic Upper an/I Loire r Teeth* 



Let us examine the upper teeth. \Ye must say, in the h'rst 

 place, that our evidence here is not nearly so complete, because a 

 lower jaw, from its thin nature, is more apt to be presented fossil 

 than an upper jaw : so that in the older rocks we meet with ten 

 lower jaws to one upper ja\v, and we cannot get the same evidence 

 as to the history of the upper jaw that we have of the lower ; but 

 although we are not able to trace the history of the upper teeth 

 with the same accuracy or degree of certainty, we have every reason 

 to think it was the same. We find the upper teeth shaped like a 

 triangle, as in Figs. 12, 13, 14, so we may imagine that the same 

 triangle which was formed in the lower jaw was formed in the upper 

 jaw, with this important difference, that in the upper jaw the base of 

 the triangle was turned outward, whereas in the lower jaw the base 

 of the triangle was turned inward (Fig. 36, No. 4). 



What I mean by this is illustrated in the accompanying figure 

 (Fig. 41, A J), which is an epitome of the whole history. The 

 upper teeth are represented solid, the lower teeth as hollow circles. 



In A we see a row of single cusps, the lower somewhat inside 

 of the upper. In B the lateral cusps are added. In C they are 

 enlarged. In I) the cusps are pushed outward and inward into 

 triangles. In E a spur is added on the lower molar triangle, which 

 in F and G grows out into a broad heel. In H and / a spin- 

 appears upon the upper molar triangle, and in J this causes the 

 lower molar triangle to lose its anterior cusp. Nature has corrected 

 any possible interference between these triangles in a simple manner 

 by turning the base of the triangle of the upper molars outward 

 towards what you call the buccal side. In the lower jaw, on the 

 other hand, the base of the triangle is turned inward to the lingual 

 side, so that finally we have the two triangles alternating, coming 

 together as in D and making a beautiful cutting mechanism ; because 

 if any food gets in between these triangular shears the food tends to 

 press these teeth forward and backward, therefore crowding the teeth 

 more closely together and tending to tighten and improve the shear, 

 whereas if the teeth were placed in line, as in C, and food were 

 to get in between, the effect would be to crowd the two jaws apart 

 and lessen the exact cutting power of the shear. 



Now we see that we can compare the lower and upper triangles 

 to each other. How about the heels or spurs, and why were 

 they developed ? They were developed because these animals required 



* [Other and more recent views as to the mechanical relations of the evolving tooth 

 parts are presented on pp. 61*, 68*, 82*, Figs. 208, 209. ED.] 





